We can see that the MRI Ruby version doubled the time for this running, while the JRuby version only increased like 30% the running time.

More importantly a look at the Activity Monitor shows that the CPU usage is 100% for the MRI Ruby running process while for the JRuby process the CPU usage shows 200% usage. Meaning that in the first case, even when it is multithreaded, only 1 thread is executing at any given time, not taking advantage of the two cores.
The JRuby version in contrast takes full advantage of the dual core using the full CPU power. This is because JRuby uses the Thread model provided by the Java Runtime.

There is a way more comprehensive comparison in the following blog: http://blog.headius.com/2009/04/how-jruby-makes-ruby-fast.html. It has great explanations of how to tweak JRuby for performance to make it faster.